The origins of Marvel heroes the Scarlet Witch and Quicksilver are up in the air, both in comics and in the Marvel Cinematic Universe, but Marvel have just given us a big piece of the puzzle.
Marvel stealth released a one-shot digital comic, Avengers: Age of Ultron Prelude - This Scepter'd Isle, which follows Loki's scepter from the finale of the first Avengers movie and into the hands of Baron Von Strucker in the credits scene of Captain America: The Winter Soldier. The one-shot is written by Will Corona Pilgrim, and illustrated by Wellinton Alves. It's revealed that Strucker, acting on behalf of Hydra, used the scepter on test subjects in the fictional European city of Sokovia to "unlock" any powers they might have.
And Wanda and Pietro are two of the test subjects.
This is an interesting twist on the origins of these two Avengers in comics, at the same time as their comics origin is being twisted in Avengers & X-Men: Axis and now in Uncanny Avengers. The term the Hydra doctor uses in this MCU tie-in comic - "unlocking" - is interesting, as if their powers were inherent but submerged - an idea very similiar to Marvel Comics' recent "Inhumanity" event which saw latent Inhumans' powers activated by an outside force, in that case a Terrigen bomb.
We previously speculated about this possibility back in November, threading in the events of Agents of S.H.I.E.L.D.'s revelations about Inhumans with Wanda and Pietro's debut in the final moments of Captain America: The Winter Soldier to speculate that Loki's scepter could be powered by a Terrigen Crystal. Remember, the stone in Loki's sceptre is blue - the same color as Terrigen crystals and mists in comics.
Also, the thought that the crystal in Loki's Scepter could be an Infinity Stone has some added credence, as a S.H.I.E.L.D. scientist in the early part of this one-shot comic remarks the crystal and the Tesseract are connected.